1. Field of the Invention
This invention relates to a tool clamping mechanism which clamps a tool by pulling a stud thereof inward via a front end of a spindle.
2. Description of the Related Arts
A machine tool such as a machining center which can automatically exchange tools includes a tool clamping mechanism in its spindle so as to firmly hold a detachable tool therein against a force applied during a cutting, drilling or other operation thereof. In order to firmly hold the tool, such a clamping mechanism has a plurality of coned disc springs arranged in series. This is not only because the clamping mechanism has to generate an urging force of 1000 kgf to 3000 kgf for firmly holding the tool during its operation but also because the coned disc springs should be axially flexed by an amount of 20 mm to 30 mm when the tool is removed therefrom. Thus, a cylinder piston mechanism is required to provide a large thrust, which means that the spindle of the machine tool inevitably becomes large. Further, compressing the coned disc springs requires a large force and a long time. Further, it takes a long time to exchange the tool. In order to allow the coned disc springs to be deformed when they are flexed, clearances should be secured between a tool pulling shaft fitted through the coned disc springs and inner circumferences of the coned disc springs, and between the outer surfaces of the coned disc springs and the spindle. Thus, when the spindle is rotated, the coned disc springs move within the foregoing clearances. This prevents a reliable and high speed rotation of the spindle.
A variety of proposals have been made for overcoming the foregoing problems of the prior art. For instance, FIG. 1 of the accompanying drawings shows a tool clamping mechanism which includes a tool pulling shaft 102 fitted into a rotatable spindle 101 in order to firmly hold a tool. Further, the tool clamping mechanism includes a plurality of coned coil springs 103 for generating a strong pulling force to draw the tool pulling shaft 102 against a force applied to the tool, and a coil spring 104 having a weak force necessary only for supporting a weight of the tool. In response to a tool exchanging command at the end of a machining operation, a thick cylinder piston 105 pushes a sleeve 106 via a rod 105a, thereby compressing the coned disc springs 103. In order to exchange the tool, a thin cylinder piston 107 pushes the tool pulling shaft 102, compresses the coil spring 104, and moves the tool pulling shaft 102 as required. Two kinds of springs are used, and the stronger one is compressed prior to exchanging the tool. The other spring is then compressed so as to exchange the tool quickly.
Referring to FIG. 2, Japanese Utility Model Laid-Open Publication No. Hei 2-104906 describes a tool clamping mechanism which includes a known force increasing mechanism for reducing the thrust of a cylinder piston necessary for compressing coned disc springs. A force of coned disc springs 109 surrounding a tool pulling shaft 108 is transmitted to the tool pulling shaft 108 via a tapered sleeve 110 and balls 111. In this arrangement, the balls 111 come into contact with a surface of the tapered sleeve 110, and a tapered surface of a spindle 114, thereby increasing a force for the coned disc springs 109 to pull the tool. Thus, the tool can be pulled using the force of the coned disc springs 109 which is weaker than the tool pulling force. Further, it is possible to reduce the thrust of the cylinder piston when clamping the tool. This tool clamping mechanism is effective in making the cylinder piston compact and reducing the tool exchange time. Other tool clamping mechanisms with force increasing mechanisms are also disclosed in Japanese Patent Laid-Open Publications No. Sho 57-21205 and No. Hei 4-41108, and Japanese Utility Model Laid-Open Publication No. Hei 3-40495.
Japanese Patent Laid-Open Publication No. Hei 1-193106 shows and describes a tool clamping mechanism in which no urging force is applied to a tool pulling shaft from coned disc springs when replacing a tool. Referring to FIG. 3, a coned disc spring nut 114 is fixed to a rear end of a first rod 113. Coned disc springs 116 are present between the coned disc spring nut 114 and a coned disc spring support 115 which confront with each other. A second rod 117 behind the first rod 113 advances to push a lock ring 118 forward against a force of a coil spring 119, thereby moving lock balls 120 inward. Thus, the spring support 115 becomes axially movable. When the second rod 117 is withdrawn, the lock rings 118 cause the lock ball 120 to move outward. Thereafter a axial movement of the spring support 115 is restricted. In order to release the clamped tool, the cylinder piston 121 pushes the second rod 117 forward, moves the lock ball 120 inward, enables the first rod 113 and the coned disc springs 116 to move, and prevents the urging force of the coned disc spring from being transmitted to the coned disc spring nut 114.
The tool clamping mechanism of FIG. 1 should have the cylinder piston with a large thrust in order to compress the coned disc springs 103, and the cylinder piston for the coil spring 104. This inevitably enlarges and complicates the tool clamping mechanism.
Referring to FIG. 2, the tool clamping mechanism can reduce the thrust of the cylinder piston, but cannot reduce a stroke of the cylinder piston. This is not effective in down-sizing the tool clamping mechanism. Further, an extent by which the coned disc springs 109 are flexed is not reduced, so a number of coned disc springs 109 should be stacked. Thus, it is substantially impossible to overcome the problem of unreliable rotation of the spindle 114.
The tool clamping mechanism in FIG. 3 can reduce the flexed amount of the coned disc springs 116, and overcome the foregoing problem during the rotation of the spindle. However, the coned disc springs 116 are directly pushed by the cylinder piston, which should be large. Further, there is a problem that the tool may fall from the clamping mechanism at the moment the urging force for the first rod 113 is interrupted from the coned disc springs 116.